.\" Copyright (c) 1993
.\"	The Regents of the University of California.  All rights reserved.
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.\"    may be used to endorse or promote products derived from this software
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.\"	@(#)daemon.3	8.1 (Berkeley) 6/9/93
.\" $FreeBSD: src/lib/libc/gen/daemon.3,v 1.15 2007/01/09 00:27:53 imp Exp $
.\"
.Dd June 9, 1993
.Dt DAEMON 3
.Os
.Sh NAME
.Nm daemon
.Nd run in the background
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn daemon "int nochdir" "int noclose"
.Sh DESCRIPTION
The
.Fn daemon
function is for programs wishing to detach themselves from the
controlling terminal and run in the background as system daemons.
The
.Xr fork 2
system call is used; see CAVEATS below about the environment after a
.Fn fork
(without a corresponding call to one of the exec routines).
On Mac OS X, the use of this API is discouraged in favor of using
.Xr launchd 8 .
.Pp
Unless the argument
.Fa nochdir
is non-zero,
.Fn daemon
changes the current working directory to the root
.Pq Pa / .
.Pp
Unless the argument
.Fa noclose
is non-zero,
.Fn daemon
will redirect standard input, standard output, and standard error to
.Pa /dev/null .
.Sh RETURN VALUES
.Rv -std daemon
.Sh ERRORS
The
.Fn daemon
function may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr fork 2
and
.Xr setsid 2 .
.Sh SEE ALSO
.Xr fork 2 ,
.Xr launchd 8 ,
.Xr setsid 2 ,
.Xr sigaction 2
.Sh HISTORY
The
.Fn daemon
function first appeared in
.Bx 4.4 .
.Sh CAVEATS
There are limits to what you can do in the child process.
To be totally safe you should restrict yourself to only
executing async-signal safe operations (see
.Xr sigaction 2 )
until such time
as one of the exec functions is called.
All APIs, including global data symbols, in any framework or library
should be assumed to be unsafe after a 
.Fn fork
unless explicitly documented to be safe or async-signal safe.
If you need to use these frameworks in the child 
process, you must exec.
In this situation it is reasonable to exec yourself.
.Pp
Unless the
.Fa noclose
argument is non-zero,
.Fn daemon
will close the first three file descriptors and redirect them to
.Pa /dev/null .
Normally, these correspond to standard input, standard output, and
standard error.
However, if any of those file descriptors refer to something else, they
will still be closed, resulting in incorrect behavior of the calling program.
This can happen if any of standard input, standard output, or standard
error have been closed before the program was run.
Programs using
.Fn daemon
should therefore either call
.Fn daemon
before opening any files or sockets, or verify that any file
descriptors obtained have values greater than 2.
.Pp
The
.Fn daemon
function temporarily ignores
.Dv SIGHUP
while calling
.Xr setsid 2
to prevent a parent session group leader's calls to
.Xr fork 2
and then
.Xr _exit 2
from prematurely terminating the child process.
